1. Field of the Invention
This invention relates generally to processes for the separation of solid catalyst particles from gases and the stripping of hydrocarbons from catalyst. More specifically, this invention relates to the separation of catalyst and gaseous materials from a mixture thereof in a cyclonic disengaging vessel of an FCC process.
2. Description of the Prior Art
Cyclonic methods for the separation of solids from gases are well known and commonly used. A particularly well known application of such methods is in the hydrocarbon processing industry were particulate catalysts contact gaseous reactants to effect chemical conversion of the gas stream components or physical changes in the particles undergoing contact with the gas stream.
The FCC process presents a familiar example of a process that uses gas streams to contact a finally divided stream of catalyst particles and effects contact between the gas and the particles. The FCC processes, as well as separation devices used therein are fully described in U.S. Pat. Nos. 4,701,307 and 4,792,437, the contents of which are hereby incorporated by reference.
The most common method of separating particulate solids from a gas stream uses a cyclonic separation. Cyclonic separators are well known and operate by imparting a tangential velocity to a gases containing entrained solid particles that forces the heavier solids particles outwardly away from the lighter gases for upward withdrawal of gases and downward collection of solids. Cyclonic separators usually comprise relatively small diameter cyclones having a tangential inlet on the outside of a cylindrical vessel that forms the outer housing of the cyclone.
Cyclones for separating particulate material from gaseous materials are well known to those skilled in the art of FCC processing. In the operation of an FCC cyclone tangential entry of the gaseous materials and catalyst creates a spiral flow path that establishes a vortex configuration in the cyclone so that the centripetal acceleration associated with an outer vortex causes catalyst particles to migrate towards the outside of the barrel while the gaseous materials enter an inner vortex for eventual discharge through an upper outlet. The heavier catalyst particles accumulate on the side wall of the cyclone barrel and eventually drop to the bottom of the cyclone and out via an outlet and a dip leg conduit for recycle through the FCC arrangement. Cyclone arrangements and modifications thereto are generally disclosed in U.S. Pat. Nos. 4,670,410 and 2,535,140.
The FCC process is representative of many processes for which methods are sought to quickly separate gaseous fluids and solids as they are discharged from a conduit. In the FCC process one method of obtaining this initial quick discharge is to directly connect a conduit containing a reactant fluid and catalyst directly to a traditional cyclone separators. While improving separation, there are drawbacks to directly connecting a conduit discharging a mixture of solids and gaseous fluids into cyclone separators. Where the mixture discharged into the cyclones contains a high loading of solids, direct discharge requires large cyclones. In addition, instability in the delivery of the mixture may also cause the cyclones to function poorly and to disrupt the process where pressure pulses cause an unacceptable carryover of solids with the vapor separated by the cyclones. Such problems are frequently encountered in processes such as fluidized catalytic cracking. Accordingly, less confined systems are often sought to effect an initial separation between a mixture of solid particles and gaseous fluids.
U.S. Pat. Nos. 4,397,738 and 4,482,451, the contents of which are hereby incorporated by reference, disclose an alternate arrangement for cyclonic separation that tangentially discharges a mixture of gases and solid particles from a central conduit into a containment vessel. The containment vessel has a relatively large diameter and generally provides a first separation of solids from gases. This type of arrangement differs from ordinary cyclone arrangements by the discharge of solids from the central conduit and the use of a relatively large diameter vessel as the containment vessel. In these arrangements the initial stage of separation is typically followed by a second more compete separation of solids from gases in a traditional cyclone vessel.
In addition to the separation of the solid catalyst from the gases, effective operation of the FCC process also requires the stripping of hydrocarbons from the solid catalyst as it passes from the reactor to a regenerator. Stripping is usually accomplished with steam that displaces adsorbed hydrocarbons from the surface and within the pores of the solid catalytic material. It is important to strip as much hydrocarbon as possible from the surface of the catalyst to recover the maximum amount of product and minimize the combustion of hydrocarbons in the regenerator that can otherwise produce excessive temperatures in the regeneration zone.
U.S. Pat. No. 4,689,206 discloses a separation and stripping arrangement for an FCC process that tangentially discharges a mixture of catalyst and gases into a separation vessel and passes gases upwardly from a lower stripping zone into a series of baffles for displacing hydrocarbons from the catalyst within the separation vessel. While the arrangement shown in U.S. Pat. No. 4,689,206 may effect some stripping of hydrocarbon gases from the catalyst in the separation vessel, the arrangement does not utilize all of the available gases for stripping of the hydrocarbons in the separation vessel and does not distribute the stripping gas that enters the separation vessel in a manner that insures its effective use via good dispersion within the catalyst phase.
While it is beneficial to effect as much stripping and recover as many hydrocarbons as possible from FCC catalyst, refiners have come under increasing pressure to reduce the amount of traditional stripping medium that are used to effect stripping. The pressure stems from the difficulty of disposing the sour water streams that are generated by the contacting the catalyst with steam in typical stripping operations. Therefore, while more efficient process operations call for the use of more effective hydrocarbon stripping from FCC catalyst, the quantities of the preferred stripping mediums are being restricted.